1. Field of the Invention
The present invention relates in general to a pulse compressor for compressing many-cycle femtosecond-duration high-energy pulses to near-single-cycle durations using a single quadratic nonlinear crystal.
2. Description of the Background Art
High-energy, short-pulse sources are needed to study high-intensity interactions of light with matter, a fact which has driven the development of pulse compression techniques to improve upon the limits of high-power lasers. Noble-gas filled hollow waveguide compressors can generate near-single-cycle pulses with energies of about a millijoule. Recently another approach has emerged, where the waveguide is replaced by the self-guiding of filamentation in gas. The new technique has many advantages over the old, but shares its limitation to energies of a few millijoules, due to multiple filamentation at powers much above the critical power for self-focusing.
In contrast, compression based on self-defocusing nonlinearities in quadratic media has no fundamental limit to its scalability. It is simple and efficient. It employs the cascaded quadratic nonlinearity (χ(2):χ(2)), a phenomenon which produces Kerr-like nonlinear phase shifts of controllable sign and magnitude. Negative nonlinear phase shifts generate new frequency components, which are brought into phase with one another by normal dispersion, either in a subsequent stage, or by soliton effect (i.e., with propagation ended part-way through the first N-soliton period). Because the propagation is spatially unguided, scaling to higher energies simply requires increasing the beam size in order to maintain appropriate intensity. Thus, energy-scaling is limited only by the availability of suitably large-aperture quadratic nonlinear crystals.
At present, single crystals are commercially available that can accommodate pulse energies up to ˜100 millijoules for self-defocusing based compression. Higher energies may possibly be accommodated by engineering two-dimensional crystal arrays. To date, however, the method has been limited to the generation of many-cycle pulses. This limitation is due to the group-velocity mismatch (GVM) between the fundamental (FF) and second-harmonic (SH) fields coupled in the nonlinear process. Until now, GVM has been seen as an insurmountable obstacle to generating few-cycle pulses with quadratic nonlinear crystals.